Stoned Drivers: The Case Against Panic

Pot prohibitionists undermine their own warnings about legalization and car crashes.

If marijuana is legalized, John Mica warned at a recent congressional hearing, there will be blood. "In the last dozen years," the Florida Republican said, "we've had [half] a million Americans slaughtered on the highways…and half of those fatalities are related to people who are impaired through alcohol or drugs." Legal pot will compound this "phenomenal devastation," he said, since "we are going to have a lot more people stoned on the highway."

Mica, a proud pot prohibitionist who chairs a subcommittee of the House Oversight and Government Reform Committee, convened the hearing to raise an alarm about the deadly threat that legalization poses to anyone navigating the roads and highways. But by the end of the hearing, anyone who was paying attention recognized that his grim prophecies have little basis in fact.

Start with the "phenomenal devastation" that Mica described. When asked how many of those deaths can be attributed to marijuana, Jeff Michael, associate administrator for research and program development at the National Highway Traffic Safety Administration (NHTSA), replied, "That's difficult to say….We don't have a precise estimate." The most he was willing to affirm was that the number is "probably not" zero.

Michael was much more confident on the subject of alcohol-related traffic deaths. In 2012, he said, based on data from NHTSA's Fatality Analysis Reporting System (FARS), "10,322 people died in crashes in which the driver had a blood-alcohol [level] above the legal limit." NHTSA counted 33,561 traffic fatalities that year, so accidents in which alcohol probably was a factor accounted for 31 percent of the total.

What about marijuana? A 2010 NHTSA analysis found that 18 percent of fatally injured drivers tested positive for drugs other than alcohol in 2009. A total of 3,952 drivers tested positive for psychoactive substances that included prescription and over-the-counter medications as well as illegal drugs such as marijuana, cocaine, heroin, and PCP. That number is often mistakenly cited as an estimate of deaths caused by drugged driving each year. But some of those drivers also tested positive for alcohol, and even those who had not been drinking were not necessarily impaired by drugs. As NHTSA warned, "drug involvement does not mean the driver was impaired or that drug use was the cause of the crash."

That point is especially important when it comes to marijuana, since the test results used by FARS indicate the presence of metabolites that linger in blood and urine long after the drug's effects wear off—"up to 30 days after use" for frequent consumers, as Rep. John Fleming (R-La.) noted during Mica's hearing. Those test results indicate that the driver was a cannabis consumer, but they do not indicate whether he was under the influence at the time of the crash, let alone whether marijuana contributed to the accident. They are about as useful as an alcohol test indicating that a driver drank a beer at some point in the previous month.

You can begin to see why Michael, the NHTSA official, did not want to hazard a guess as to the number of marijuana-related traffic fatalities. Mica and Fleming, who came to make the case against legalization even though he is not a member of Mica's subcommittee, were not so cautious, repeatedly citing increases in the number of dead drivers testing positive for marijuana as evidence of a growing threat posed by drugged driving. One of the studies mentioned by Fleming, focusing on FARS data for Colorado, found that the percentage of "marijuana-positive drivers" rose from 4.5 percent in early 1994 to 10 percent at the end of 2011, a period that included the commercialization of medical marijuana in that state. But the study, which was reported in the journal Drug and Alcohol Dependence last April, included a crucial caveat:

THC metabolites are detectable in an individual's blood or urine for several days and sometimes weeks for heavy marijuana users….This study cannot determine cause and effect relationships, such as whether marijuana-positive drivers contributed to or caused the fatal motor vehicle crashes. Colorado may have an increased number of drivers, in general, who were using marijuana, not just an increase in the proportion who were involved in fatal motor vehicle crashes….The primary result of this study may simply reflect a general increase in marijuana use during this same time period in Colorado.

The two other sources cited by Fleming and Mica, the aforementioned 2010 NHTSA report and an analysis of FARS data published by the American Journal of Epidemiology in January, included similar warnings.

Notably, total traffic fatalities fell during the periods covered by all three studies, which seems inconsistent with the notion that less repressive marijuana policies make the roads more dangerous. In Colorado, the downward trend in fatal crashes has continued since marijuana was legalized for recreational use at the end of 2012. Total fatalities fell in 2013 and in the first seven months of this year.

Data from the National Survey on Drug Use and Health indicate that drugged driving is, if anything, less common than it was a decade ago. In 2012, 3.9 percent of respondents reported driving under the influence of an illegal drug in the previous year, down from 4.7 percent in 2002.

Mica's hearing highlighted another crucial gap in our knowledge about the connection between marijuana and traffic accidents. "The available evidence does not support the development of an impairment threshold for THC which would be analogous to that for alcohol," Michael testified. "With alcohol, we have a considerable body of evidence that can place risk odds at increasing levels of blood alcohol content. For example, 0.08 [percent] blood alcohol content is associated with about four times the crash risk of a sober person….Beyond some broad confirmation that higher levels of THC are generally associated with higher levels of impairment, a more precise association of various THC levels and degrees of impairment [is] not yet available."

If so, you might wonder, what is the basis for seemingly precise and scientific standards such as the one recently adopted by Colorado and Washington, which says a driver is guilty of driving under the influence of a drug (DUID) if his blood contains five or more nanograms of THC per milliliter? Critics of that rule wonder the same thing, especially since it's clear that many regular cannabis consumers can drive safely at levels far above five nanograms. The best that can be said for the five-nanogram cutoff is that it's preferable to a "zero tolerance" standard, which essentially makes it illegal for all pot smokers to drive, whether they are impaired or not.

There is a similar problem with roadside saliva tests, which Mica talked up at the hearing as a possible solution to the problem of identifying stoned drivers. According to Mica, the tests "can tell you if anyone has used marijuana within four hours." But as Rep. Gerry Connolly (D-Va.) noted, citing NHTSA, "Marijuana intoxication is short-lived. Peak acute effects following cannabis inhalation are typically achieved within 10 to 30 minutes, with the effects dissipating quickly after about an hour." So even the saliva test favored by Mica would mistakenly identify unimpaired drivers as public menaces.

Michael said NHTSA is working to fill these gaps. A before-and-after roadside survey of drivers in Washington will try to determine if the percentage under the influence of marijuana goes up after legalization. Michael said NHTSA is also working on "a new study which compares the crash risk of drivers using drugs to those with no drugs in their system," which is "the first such investigation of drug crash risk in the United States." A third study, which Michael did not mention, is using a sophisticated driving simulator to measure how different THC levels affect driving ability.

Although more research is needed, it is already clear that marijuana impairs driving less dramatically than alcohol (which is why some researchers believe that legalization might actually reduce traffic fatalities, to the extent that people substitute marijuana for alcohol). According to a 1993 NHTSA report that Connolly quoted, "The impairment [from marijuana] manifests itself mainly in the ability to maintain a lateral position on the road, but its magnitude is not exceptional in comparison with changes produced by many medicinal drugs and alcohol. Drivers under the influence of marijuana retain insight in their performance and will compensate when they can, for example, by slowing down or increasing effort. As a consequence, THC's adverse effects on driving performance appear relatively small."

Connolly tried to put the risk posed by marijuana-impaired driving in perspective, noting that alcohol, speeding, fatigue, and distractions such as cellphones and unruly children all seem to pose greater dangers. "No one's arguing that it's a good idea" to drive after smoking pot, he said, "but the fact of the matter is, we don't have a lot of data" to support the fears of prohibitionists like Mica and Fleming.

Another crucial point made by Connolly: Even if stoned drivers pose a significant threat to public safety, it does not follow that forcibly suppressing marijuana consumption is the right way to deal with that threat, any more than forcibly suppressing alcohol consumption is the right way to deal with the threat posed by drunk drivers. "Opposing prohibition of alcohol" is not "an endorsement for drunk driving," Connolly observed. Likewise, "discouraging the inappropriate use of drugs need not—perhaps should not—involve total prohibition and criminalization." I would have left out the perhaps, but otherwise he's on the right track.

This article originally appeared at Forbes.